Double faces on UV map?
Smart UV Project works for me on 2.79. I added a primitive cylinder Edit/Face mode/deleted the cylinder top face. Back to Object mode/added "Solidify Modifier" (BTW hate that name), applied "Solidify Modifier". Back to edit/face mode, select all/apply Smart UV Project at default settings. Result is fine.

So are you saying that the faces/verts of the outside of the cylinder are locked to the inside faces/verts?